Implantable cardiac stimulation device having a capture assurance system which minimizes battery current drain and method for operating the same

ABSTRACT

An improved pacing system and related method for use in an implantable pacemaker or defibrillator are disclosed which operate using a predetermined subset of combinations of possible combinations for pacing stimulus pulse amplitude and pulse width, which subset of combinations are the most energy-efficient pairs, thereby ensuring reduced battery current drain. This is accomplished by ensuring that each combination has the lowest battery charge drain of any combination having at least the rheobase value of that particular combination as a function of cardiac chronaxie. The preferred embodiment of the pacing system of the present invention includes capture verification to enable a substantially reduced safety margin to further minimize the level of battery charge drain. The preferred embodiment of the pacing system of the present invention also includes the capability to produce a safety backup pulse, to ensure that the pacing system never misses a beat. The preferred embodiment of the pacing system of the present invention further includes the capability to initially and periodically determine the stimulation threshold and cardiac chronaxie.

FIELD OF THE INVENTION

The present invention relates generally to implantable cardiacstimulation devices, and more particularly to an automatic pacingstimulus capture assurance system and related method for use in animplantable pacemaker or defibrillator.

BACKGROUND OF THE INVENTION

Pacemakers are used to treat a condition called bradycardia, in whichthe heart beats too slowly. A pacemaker system includes threecomponents—a pulse generator, at least one pacing lead, and aprogrammer. The pulse generator contains the battery and the electroniccircuitry, or “brain,” which directs the pulse generator to sendelectrical stimulation pulses through the pacing leads, stimulating theheart and causing it to beat at a controlled “normal” rhythm. The pacingleads may also be used to transmit cardiac signals (i.e., depolarizationsignals) to the pulse generator. Pacemakers may also includephysiological sensors which provide the pacemaker with an indicia ofpatient hemodynamic needs, so that the pacemaker can adjust its pacingstrategy to satisfy those needs. An external programmer is used tomonitor the operation of the pacemaker noninvasively (referred to asinterrogating the pacemaker) and to change pacemaker settings (referredto as programming the pacemaker).

Implantable cardioverter/defibrillators (ICD's) are used to treat acondition called tachycardia, in which the heart beats at a rapid,uncoordinated manner. An ICD system, like a pacemaker system, is made upof three components—an ICD pulse generator, at least one lead, and aprogrammer used to interrogate and program the ICD. The ICD pulsegenerator monitors the rhythm of the heart from the leads, andadministers an electric shock when necessary to control/terminatetachycardias and restore a normal heartbeat. ICD's also includepacemakers, since many patients needing an ICD can generally benefitfrom some pacing therapy.

Pacemaker technology has evolved rapidly over the last several decades,resulting in improvements making pacemakers more automatic in theirability to adapt to the specific needs of individual patients. ICD'shave also evolved to become increasingly sophisticated both in theirtreatment of tachycardias and in their inclusion of full-featuredpacemakers. In addition, both pacemakers and ICD's have becomesignificantly smaller with greater longevity resulting from increasinglyefficient operation which conserves battery power. It is certainlywidely appreciated by those skilled in the art that it is imperative tominimize the operating current required by these devices to achieve thetwin goals of extending their operating lives and making the device sizeas small as is possible.

One of the most significant ways of increasing the efficiency ofpacemakers, as well as the pacemaker system operation in ICD's, has beenthe development of systems which automatically and continuously adjustthe level of energy of electrical stimulation pulses delivered to pacethe patient's heart. In order for a stimulation pulse from a pacemakerto depolarize cardiac muscle tissue to cause it to contract (to cause aheartbeat), the energy of the stimulation pulse must be sufficient to“capture” the heart, that is to cause a depolarization of the atrium orventricle. The level of energy in a pacemaker stimulation pulse which isnecessary to cause capture is referred to as the stimulation thresholdlevel, or simply as the stimulation threshold. Most pacemakers and ICD'sare capable of determining the stimulation threshold in a test whichreduces stimulation pulse energy until loss of capture is detected.

If the stimulation pulse is below the stimulation threshold, capturewill not occur and the stimulation pulse will be ineffective. If, on theother hand, the stimulation pulse is at or above the stimulationthreshold, capture will most likely occur. The amount of energy in thepacemaker stimulation pulse above the stimulation threshold provides nouseful function and is wasted. For the purposes of conserving batteryenergy, and maximizing the life of the device, it is desirable to keepthe amount of energy in the pacemaker stimulation pulse or slightlyabove at the stimulation threshold.

The stimulation threshold varies widely, not only from patient topatient but for any given patient substantially over both longer andshorter periods of time. Eating and sleeping can cause about a twentypercent increase in the stimulation threshold. Posture and exercise canchange the stimulation threshold about fifteen to twenty percent.Following lead implantation, the stimulation threshold typicallyincreases to a peak level three months after implantation, and thenstabilizes at a lower level.

Since it is desirable to ensure capture, physicians typically programthe pacemaker or ICD to deliver pacemaker stimulation pulses at anenergy level substantially above the stimulation threshold. The amountthat the pacemaker stimulation pulse energy exceeds the stimulationthreshold is referred to as the “safety factor.” Physicians generallyprogram the stimulation pulse energy at a safety factor of 1.7 to 2times the stimulation threshold. It is well appreciated by those skilledin the art that the safety factor results in a substantially increasedlevel of current drain from the battery, and in reduced devicelongevity. Since patient safety is paramount, this has been a situationwhich, until a few years ago, was acceptable.

Recently, the first pacemakers with automatic capture confirmation on abeat by beat basis were developed. They combine automatic backup pulsedelivery upon loss of capture with automatic output regulation ofstimulation pulse levels to a value just above the stimulationthreshold. These devices automatically search for and locate thestimulation threshold and pace at a level just above that stimulationthreshold (for example, 0.3 volts above the stimulation threshold),typically with the amplitude or voltage level of the stimulation pulsebeing varied and the pulse width remaining constant, such as, forexample, at a nominal value of 0.5 mS. Such a stimulation thresholdsearch may be automatically done periodically, such as, for example,every eight hours.

These devices also monitor capture confirmation by looking for an evokedresponse during a window of time immediately after the stimulationpulse; for example, the window may begin 15 mS after each stimulationpulse, with the width of the window being 47.5 mS. Finally, thesedevices assume loss of capture if no evoked response is sensed duringthe window after the stimulation pulse, and, in the absence of an evokedresponse signal, provide a higher voltage (typically 4.5 volts, forexample) safety backup pulse. Thus, the device ensures that thepatient's heart never misses a beat. In the event of loss of captureindicating a change in the stimulation threshold (which may be presumed,for example, after the delivery of two consecutive backup pulses), thedevice will search again for the stimulation threshold.

One example of a system for locating the stimulation threshold is shownin U.S. Pat. No. 5,669,392, to Ljungström. Similarly, an example of acapture confirmation system is illustrated in U.S. Pat. No. 5,782,889,to Höegnelid et al. Finally, an example of a system for automaticallyproviding a backup pulse is shown in U.S. Pat. No. 5,846,264, toAndersson et al., all of which are each hereby incorporated herein byreference.

Another automatic system having similar characteristics is described inU.S. Pat. No. 5,350,410, to Kleks et al. is hereby incorporated hereinby reference. It may also be noted that these features may also beincluded in the pacemaker contained in an ICD.

The systems described above thus represent a significant enhancement topacemaker system longevity through minimization of current drain byoperating at much lower safety factors. However, they automatically varythe amplitude or voltage of the stimulation pulse without looking at ortaking into account the actual variation in current drain caused by achange in the voltage level of the stimulation pulse. Since capture is afunction of energy delivered to the cardiac tissue, a more realisticmanner of considering stimulation threshold is to view it as acontinuous function described by the strength-duration relationship.

See, for example, Stokes and Bornzin, “The Electrode-Biointerface:Stimulation”, Chapter 3 of Modern Cardiac Pacing, edited by S. SergeBarold, M.D. (Futura Publishing Co., 1985). The fundamental nature ofthe strength-duration relationship is that for very narrow pulse widths,a large stimulation pulse amplitude is required to obtain capture, andfor wide pulse widths, a lower stimulation pulse amplitude is requiredto obtain capture. Thus, there is a wide variety of pulse amplitude andpulse width combinations which may be used to obtain capture at a givenstimulation threshold. There are also a corresponding wide variety ofpulse amplitudes and pulse widths combinations which may be used toprovide an adequate safety factor.

However, the ability to independently program pulse amplitude and pulsewidth does not necessarily provide optimal pacing, because in order tomaintain a desired or adequate safety factor, battery current drain isnot considered as a primary factor. The first to note this was Bornzinin U.S. Pat. No. 5,697,956 (the '956 patent), which is assigned to theassignee of the present invention. Bornzin did not provide independentprogramming of pulse amplitude and pulse width, but rather provided onlythe programming of pulse energy, selected from a series of pulses ofincreasing or decreasing energy that had been selected to provideoptimal pacing. This basic principle is the foundation of the presentinvention as well, and U.S. Pat. No. 5,697,956 is hereby incorporatedherein by reference.

The technique taught in the '956 patent is a five step procedure whichfirst determines the patient's strength-duration curve by determining aseries of pulse thresholds for a particular patient as a function of aplurality of pulse widths. Second, desired pulse amplitude safety factoris added to the strength-duration curve to produce a plurality of pulseamplitude and pulse width combinations that would ensure capture at thedesired safety factor. This may be thought of as a second curveidentical to the strength-duration curve but displaced directly above itby an amount equal to the increase in voltage selected as a safetyfactor.

Third, a pulse current drain is computed as a function of each of thepulse amplitude and pulse width combinations determined in the secondstep. Fourth, a series of optimal pulse amplitude and pulse widthcombinations is selected that provides a minimal pacing current drain asa function of pacing energy. Fifth, the pacemaker is automaticallyprogrammed to the optimal pacing energy using the pulse amplitude andpulse width combination determined in the fourth step.

The procedure taught by the '956 patent represented a significantimprovement over the previously used technique of manually programmingthe pacemaker to a desired pulse width, determining the voltagenecessary to assure capture, and programming a higher voltage to providea safety factor. For the first time, the pacing current required by thedevice was used as a guide to determine the pulse amplitude and thepulse width used. However, the '956 patent does not provide a method ofoptimization of such a device by the use of an automatic captureconfirmation system. Nor does the system of the '956 patent trulydetermine optimal operating points, but rather it finds optimal pointsand adds a pulse amplitude (voltage) increment to such points whichhowever, does not result in establishing optimal operating points.

It is a primary feature of the present invention to determine improvedor optimal operating points from a battery current efficiencystandpoint, and to operate the device at such operating points. In thisregard, it is an objective of the present invention to determine thosepulse amplitude/pulse width combinations that have the lowest batterydrain of such combinations.

SUMMARY OF THE INVENTION

Briefly, the present invention determines improved or optimal operatingpoints (stimulation pulse amplitude and stimulation pulse widthcombinations, hereinafter pulse amplitude/pulse width combinations) froman efficiency standpoint, as measured by battery charge drained from thebattery. These operating points vary widely in the pacing energy thatthey deliver to cardiac tissue, and, as such, offer a complete array ofpulse amplitude/pulse width combinations suitable to meeting the pacingthresholds of virtually any patient. Following the determination ofthese operating points, the pacing system is operated such that it pacesat these operating points.

In one embodiment, the determination of the operating points is made bydetermining which of all of the possible pulse amplitude/pulse widthcombinations have the lowest battery charge drain at a predeterminedlevel of stimulation efficacy. The level of stimulation efficacyrequired varies from patient to patient, and will also vary in the samepatient over a period of time. The objective is thus to determine andutilize a quantifiable measure of stimulation efficacy.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention are best understoodwith reference to the drawings, in which:

FIG. 1 is a functional block diagram of a pacemaker system in which thecapture assurance system of the present invention can be implemented,showing a pacemaker, a programmer, and two pacing leads which areimplanted in a heart;

FIG. 2 is a strength-duration curve which has pulse duration plotted onthe horizontal coordinate, and pulse amplitude plotted on the verticalcoordinate;

FIG. 3 is an array containing battery charge drain values associatedwith various pulse amplitudes and pulse widths;

FIG. 4 is an array correlating the array of shown in FIG. 3 whichcontains the rheobase voltage values Vb associated with the variouspulse amplitudes and pulse widths for a tissue chronaxie c with a valueof 0.4 milliseconds, with the highlighted rheobase values representingstimulation pulse settings with the minimum charge for a given rheobasevoltage value;

FIG. 5 is an array correlating to the array illustrated in FIG. 3 whichcontains the rheobase voltage values Vb associated with the variouspulse amplitudes and pulse widths for a tissue chronaxie c with a valueof 0.2 milliseconds, with the highlighted rheobase values representingstimulation pulse settings with the minimum charge for a given rheobasevoltage value;

FIG. 6 is an array correlating to the array illustrated in FIG. 3 whichcontains the rheobase voltage values Vb associated with the variouspulse amplitudes and pulse widths for a tissue chronaxie c with a valueof 0.6 milliseconds, with the highlighted rheobase values representingstimulation pulse settings with the minimum charge for a given rheobasevoltage value;

FIG. 7 is a table containing the highlighted rheobase voltage values Vbfrom the array of FIG. 4 from 0 to 7.5v for a tissue chronaxie c with avalue of 0.4 milliseconds, together with the various pulse amplitudesand pulse widths each of the rheobase voltage values Vb is associatedwith;

FIG. 8 is a flow chart which provides an overview of a method which maybe used by the present invention to manually select a value of thechronaxie c and program a corresponding sequence of optimal programpulse amplitudes and pulse widths into the pacemaker memory;

FIG. 9 is a flow chart which provides an overview of a method which maybe used by the present invention to manually select a value of thechronaxie c and program that value into the pacemaker, which selects acorresponding sequence of optimal program pulse amplitudes and pulsewidths stored in the pacemaker memory;

FIG. 10 is a flow chart which provides an overview of a method which maybe used by the present invention to calculate a value of the chronaxie cand program a corresponding sequence of optimal program pulse amplitudesand pulse widths into the pacemaker memory;

FIG. 11 is a flow chart which provides an overview of a method which maybe used by the present invention to calculate a value of the chronaxie cand program that value into the pacemaker, which selects a correspondingsequence of optimal program pulse amplitudes and pulse widths stored inthe pacemaker memory; and

FIG. 12 is a flow chart which provides an overview of the method used bythe present invention to automatically determine the optimal pacingenergy, where “optimal” means a pacing energy designed to assure adesired safety factor while minimizing battery current drain.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the present invention will be describedherein in exemplary fashion with reference to a dual-chamber pacemaker.While a dual-chamber device has been chosen for teaching purposes, itwill at once be recognized by those skilled in the art that the presentinvention could also be implemented into a single-chamber pacemaker, ora single-chamber or dual-chamber ICD having a pacemaker incorporatedtherein.

Referring first to FIG. 1, a simplified block diagram of the circuitryneeded for a dual-chamber pacemaker system is illustrated. The systemincludes a dual-chamber pacemaker 30, a ventricular pacing lead 32 andan atrial pacing lead 34, which leads are implanted with their distalportions placed in the respective chambers of a heart 36, and anexternal programmer 38 which is used to interrogate and program thepacemaker 30.

The ventricular pacing lead 32 has at least one electrode 40 which is incontact with one of the ventricles of the heart 36, and the atrialpacing lead 34 has at least one electrode 42 which is in contact withone of the atria of the heart 36. The leads 32 and 34 are electricallyand physically connected to the pacemaker 30 through a connector 44which forms an integral part of the device housing or “can” in which thecircuitry and other components of the pacemaker 30 are housed.

The connector 44 is electrically connected to a protection network 46,which is used to electrically protect the circuits within the pacemaker30 from excessive shocks or voltages which may appear on the electrodes40 and/or 42 from a defibrillation shock.

The leads 32 and 34 carry stimulating pulses to the electrodes 40 and42, respectively, from a ventricular pulse generator 48 and an atrialpulse generator 50, respectively. Further, electrical signals from theventricle of the heart 36 are sensed by the electrode 40, and passthrough the ventricular pacing lead 32 as an input to a ventricularchannel sense amplifier 52, where they are amplified. The amplifiedsignals from the ventricle are then supplied to a ventricular filter 54,where they are bandpass filtered.

Similarly, electrical signals from the atrium of the heart 36 are sensedby the electrode 42, and pass through the atrial pacing lead 34 as aninput to an atrial channel sense amplifier 56, where they are amplified.The amplified signals from the atrium are then supplied to an atrialfilter 58, where they are bandpass filtered.

The electrical signals from the ventricle are also supplied as an inputto a ventricular IEGM (intracardiac electrogram) amplifier 60, whichamplifies them. The ventricular IEGM amplifier 60 may be a digitalamplifier, providing the digitized ventricular IEGM as an output. Theventricular IEGM typically may be telemetered out of the pacemaker 30 tothe programmer 38, where it may be viewed. The ventricular IEGMamplifier 60 may also be configured to detect an evoked ventricularresponse from the heart 36 in response to an applied stimulation pulse,thereby aiding in the detection of ventricular capture.

The electrical signals from the atrium are also supplied as an input toan atrial IEGM (intracardiac electrogram) amplifier 62. The atrial IEGMamplifier 62 may be a digital amplifier, providing the digitized atrialIEGM as an output. The atrial IEGM typically may be telemetered from thepacemaker 30 to the programmer 38, where it may be viewed. The atrialIEGM amplifier 62 may also be configured to detect an evoked atrialresponse from the heart 36 in response to an applied stimulation pulse,thereby aiding in the detection of atrial capture.

The filtered, amplified signals from the ventricle (from the ventricularfilter 54), the filtered, amplified signals from the atrium (from theatrial filter 58), the ventricular IEGM (from the ventricular IEGMamplifier 60), and the atrial IEGM (from the atrial IEGM amplifier 62)are all supplied as inputs to digital control and timing circuits 64.The digital control and timing circuits 64 are also connected to drivethe ventricular pulse generator 48 and the atrial pulse generator 50.The digital control

A battery 66 supplies electrical power to the digital control and timingcircuits 64 and to a current measurement circuit 67 which is coupled tovoltage multiplier 68, as well as to other circuitry within thepacemaker 30. The voltage multiplier 68 is controlled by the digitalcontrol and timing circuits 64, and supplies either one, two, or threetimes the voltage of the battery 66 (nominally 2.8 Volts) to theventricular pulse generator 48 and the atrial pulse generator 50. Thisenables the ventricular pulse generator 48 and the atrial pulsegenerator 50 to provide pulse amplitudes greater than the batteryvoltage (typically the maximum pulse amplitude is approximately 7.5Volts).

The digital control and timing circuits 64 generate pulse amplitude,pulse width, and trigger signals that are sent to the ventricular pulsegenerator 48 and the atrial pulse generator 50 for controlling the shapeof the stimulation pulse provided by such generators. The triggersignals initiate the generation of stimulation pulses by the ventricularpulse generator 48 and the atrial pulse generator 50. The digitalcontrol and timing circuit 64 can also monitor the resulting pacemakerbattery current drain as measured by the battery current measurementcircuit 67, which includes the house keeping current as well as thecurrent used in the pacing of the atrial and ventricular chambers of theheart. The resultant current measurement is available to themicroprocessor 74 and the programmer 38.

During the time that either a ventricular pulse or an atrial pulse isbeing delivered to the heart 36, the corresponding amplifier (theventricular channel sense amplifier 52 or the atrial channel senseamplifier 56) is typically disabled by way of a blanking signalpresented to these amplifiers from the digital control and timingcircuits 64. This blanking action prevents the ventricular channel senseamplifier 52 and the atrial channel sense amplifier 56 from becomingsaturated from the relatively large stimulation pulses that are presentat their input terminals during this time. This blanking action alsohelps prevent residual electrical signals present in the muscle tissueas a result of the pacemaker stimulation from being interpreted asP-waves or R-waves.

In the preferred embodiment, the pacemaker 30 is controlled by amicroprocessor circuit 70 which is programmed to carry out control andtiming functions. The microprocessor circuit 70 is coupled to thedigital control and timing circuits 64 with a data communication bus 72.The microprocessor circuit 70 typically includes a microprocessor 74,microprocessor control circuitry 76, and an on-board RAM 78 and anonboard ROM 80. The microprocessor control circuitry 76 typicallyincludes an on-board system clock. The microprocessor circuit 70 istypically fabricated using a custom low-power microprocessor andstandard or custom RAM and ROM components.

Also controlled by the microprocessor 74 are an off-board RAM 82 and anoff-board ROM 84, both of which are coupled to the microprocessorcircuit 70 by the data communication bus 72. The offboard RAM 82 can bea single RAM element, or it may be several discrete RAM elements, eachof which is coupled to the data communication bus 72. Similarly, theoff-board ROM 84 can be a single ROM element, it is may also be severaldiscrete ROM elements, each of which is coupled to 72. The off-board RAM82 may also be subdivided into as many different memory blocks orsections (addresses) as needed in order to allow desired data andcontrol information to be stored.

The off-board RAM 82, and/or the on-board RAM 78, allow certain controlparameters to be programmably stored and modified, as required, in orderto customize the pacemaker's operation to suit the needs of a particularpatient. Further, data sensed during the operation of the pacemaker maybe stored in the off-board RAM 82, and/or the on-board RAM 78, for laterretrieval and analysis.

Representative of the types of control systems which may be used withthe present invention is the microprocessor-based control systemdescribed in U.S. Pat. No. 4,940,052, to Mann, et al. Reference is alsomade to U.S. Pat. No. 4,712,555, to Thornander, et al. and U.S. Pat. No.4,944,298, to Sholder, wherein a state-machine type of operation for apacemaker is described, and U.S. Pat. No. 4,788,980, to Mann, et al.wherein the various timing intervals used within the pacemaker and theirinter-relationship are more thoroughly described. U.S. Pat. Nos.4,940,052, 4,712,555, 4,944,298, and 4,788,980 are hereby eachincorporated herein by reference.

A clock circuit 86, which is typically a crystal-controlled oscillator,provides main timing clock signals to the digital control and timingcircuits 64, with the clock signals being supplied to other circuitsmaking use of them through the data communication bus 72.

RF (radio frequency) signals are exchanged between the pacemaker 30 andthe programmer 38, and are received and transmitted by the pacemaker 30using an antenna 88 which is connected to telemetry circuitry 90, whichis in turn connected to the digital control and timing circuits 64.

Advantageously, by using the programmer 38, desired commands may betelemetrically sent to the pacemaker 30, and data such as deviceparameters and device history from the pacemaker 30 may be received anddisplayed by the programmer 38.

The telemetry circuitry 90 may be of conventional design, such as isdescribed in U.S. Pat. No. 4,944,299, to Silvian, or as is otherwiseknown in the art. U.S. Pat. No. 4,944,299 is hereby incorporated hereinby reference.

The pacemaker 30 further includes a magnet detector 92, which provides asignal indicating when a magnet is placed over the pacemaker 30. Themagnet may be used by a physician, other medical personnel, or thepacemaker patient to perform various reset functions of the pacemaker30, and/or to indicate that a telemetry wand 94 of the programmer 38 isin place to receive data from, or send data to the pacemaker 30.

As needed for certain applications, the pacemaker 30 may further includeat least one sensor 96 which is operatively connected to the digitalcontrol and timing circuits 64. A common type of sensor is an activitysensor, such as an accelerometer, which may be mounted on the circuitryof the pacemaker 30, or a piezoelectric crystal, which may be mounted tothe case of the pacemaker 30.

Other types of sensors are also known, such as sensors which senserespiration rate, the oxygen content of blood, the pH of blood, thetemperature of blood, the QT interval, body motion, and the like. Anysensor or combination of sensors capable of sensing a physiological orphysical parameter relatable to the rate at which the heart should bebeating (i.e., relatable to the metabolic need of the patient) can beused. Such sensors are commonly used with “rate-responsive” pacemakersin order to adjust the pacemaker rate (the frequency of delivery of thestimulation pulses) in a manner which tracks the physiological ormetabolic needs of the patient.

Referring now to the programmer 38, the telemetry wand 94 is connectedto programmer telemetry circuitry 98. The programmer telemetry circuitry98 is in turn operatively connected to programmer control circuitry 100,which includes memory therein. A touch screen display 102 is operativelyconnected to the programmer control circuitry 100 to both displayinformation from the programmer control circuitry 100 and to provideinput signals to the programmer control circuitry 100.

A keyboard 104 is operatively connected to the programmer controlcircuitry 100 to provide input signals to the programmer controlcircuitry 100, and a printer 106 is operatively connected to theprogrammer control circuitry 100 to provide a permanent record of deviceparameters and operations. A more complete exemplary description of aprogrammer is given in U.S. Pat. No. 4,809,697, to Causey, III, et al.,which patent is hereby incorporated herein by reference.

The principle which the capture assurance pacing system of the presentinvention utilizes is that of determining a set of optimal pulseamplitudes and pulse widths, which are defined by determining the pairsof pulse amplitude and pulse width which provide the greatest rheobasevoltage at minimum charge drain from the battery 66. The selected pairsof pulse amplitude and pulse width are then organized into a table forlater retrieval and use; only those pairs of pulse amplitude and pulsewidth which are listed in the table will be used in the operation of thedevice.

In order to understand the principles of the present invention, it isnecessary to fully understand the parameters associated with thestrength-duration curve. Referring now to FIG. 2, a strength-durationcurve is illustrated with pulse duration plotted on the ordinate, andpulse amplitude plotted on the abscissa. The strength-duration curve isa graphical representation of the relationship between pulse amplitudeand pulse width to achieve capture.

There exists a point b on the strength-duration curve that is called therheobase, which represents the lowest pulse amplitude at which capturewill occur, with no further improvement being obtained by a furtherincrease in pulse width. Thus, in FIG. 2, it may be seen that therheobase b is approximately 0.5 Volts.

At another point on the strength-duration curve, the pulse amplitude istwo times the value of the rheobase b. The pulse width associated withsuch point on the strength-duration curve is known as the chronaxie orchronaxie time (which is represented by the character c). For thestrength-duration curve of FIG. 2, it is observed that the chronaxie cis approximately 0.4 milliseconds. Typical values for the chronaxie cfor pacing range from 0.2 milliseconds to 0.6 milliseconds.

For purposes of the following discussion it is assumed that the pacingpulse can be approximated by a rectangular pulse having an amplitude Vand a duration of d. Other waveforms can be used without departing fromthe spirit of the invention. By establishing values for a given patientfor the rheobase b and the chronaxie c, the entire strength-durationcurve may be computed by using the Weiss-Lapicque strength-durationmathematical description:

 V=b*(1+c/d)  (1)

where b is the rheobase, c is the chronaxie, V is the pulse amplitude,and d is the pulse width or duration. By solving for the rheobase b, theequation may be rewritten as:

b=V/(1+c/d)  (2)

This relationship is one of two which are key to the capture assurancepacing system of the present invention.

The other relationship which is key to the present invention is theequation relating the amount of charge delivered from the battery 66 tothe patient:

Q=(V/R)*d(for V≦2.5V)  (3)

Q=2*(V/R)*d(for 2.5V<V≦5.0V)  (4)

Q=3*(V/R)*d(for 5.0V<V≦7.5V)  (5)

where Q is the amount of charge delivered, R is the combined impedanceof the lead and cardiac tissue, V is the pulse amplitude, and d is thepulse width or duration. The third equation is used when the pulseamplitude to be delivered is low enough to be delivered directly fromthe battery 66 voltage, the fourth equation is used when the pulseamplitude requires the voltage multiplier 68 (illustrated in FIG. 1) todouble the voltage of the battery 66, and the fifth equation is usedwhen the pulse amplitude requires the voltage multiplier 68 to triplethe voltage of the battery 66. The lead/tissue impedance R typicallyranges from 200 Ohms to 1000 Ohms, with 500 Ohms being a nominal value.

The next step in understanding the operation of the capture assurancepacing system of the present invention involves establishing two tables,one table being a tabulation of charge delivered as a function of pulseamplitude and pulse width and the second table being a tabulation ofrheobase as a function of pulse amplitude and pulse width.

An example of the first type of table is shown in FIG. 3. Note that FIG.3 shows pulse amplitudes varying up to 7.5 Volts, in which case thevoltage multiplier 68 would have capability to only triple the voltageof the battery 66. For the table illustrated in FIG. 3, the lead/tissueimpedance R is assumed to be 500 Ohms.

An example of the second type of table is illustrated in FIG. 4. For thetable illustrated in FIG. 4, the chronaxie c is assumed to be 0.4milliseconds. As in the case of the table of FIG. 3, the table of FIG. 4shows pulse amplitudes varying only up to 5.0 Volts.

The tables of FIGS. 3 and 4 are then compared to determine a sequence ofpulse amplitude/pulse width combinations which progressively increase inboth rheobase value and stimulation pulse energy content. Each of thepulse amplitude/pulse width combinations in the sequence must have thelowest battery charge drain of any pulse amplitude/pulse widthcombination having at least the rheobase value of that particular pulseamplitude/pulse width combination.

By repeating this comparison for each pulse amplitude/pulse width pair,and by removing any pairs which require a higher amount of charge Q thananother pair delivering a higher rheobase b for the same or less chargeQ, all of the pairs of data points may be selected. In FIGS. 3 and 4,the pairs of data points so selected are shown in shaded areas.

FIG. 5 shows the values of the rheobase b for all of the varying pulseamplitudes and pulse widths which can be delivered with the value of thechronaxie c assumed to be 0.2 milliseconds. FIG. 5 shows pulseamplitudes varying up to 5.0 Volts. The optimal pairs of data points forthis value of the chronaxie c are shown in shaded areas in FIGS. 5 and6.

Similarly, FIG. 6 shows the values of the rheobase b for all of thevarying pulse amplitudes and pulse widths which can be delivered withthe value of the chronaxie c assumed to be 0.6 milliseconds. FIG. 6shows pulse amplitudes varying up to 5.0 Volts. The optimal pairs ofdata points for this value of the chronaxie c are shown in shaded areasin FIG. 6.

FIGS. 3 and 4 show pulse amplitudes varying up to 7.5 Volts, in whichcase the voltage multiplier 68 would have capability to treble thevoltage of the battery 66. FIG. 3 shows a plot of the charge Q deliveredfrom the battery 66 for all of the varying pulse amplitudes and pulsewidths. FIG. 4 shows a plot of the values of the rheobase b for all ofthe varying pulse amplitudes and pulse widths which can be deliveredwith the value of the chronaxie c assumed to be 0.4 milliseconds. Theoptimal pairs of data points for this value of the chronaxie c are shownin shaded areas in FIGS. 3 and 4.

In practice, it has been determined that the optimal pairs of datapoints begin in the upper left corner of the tables at a pulse width(PW) of 0.2 milliseconds, and a pulse amplitude (PA) of 0.5 Volts. Thiscombination of minimum values represents the minimum battery chargedrain (0.2 microCoulombs) for each stimulation pulse. Referring to FIGS.3 and 4 for exemplary purposes, the progression of data points goes downthe left-most column to the data point at 0.2 milliseconds, 2.5 volts(1.0 microCoulombs for a 0.83 Volt rheobase). The 2.5 volt valuerepresents the highest voltage obtainable from the battery 66 withoutusing the voltage multiplier (since pacemaker batteries typically have anominal voltage which is sufficient to generate this pulse amplitudewithout the use of a voltage multiplier). Also included in FIGS. 3 and 4in serial fashion, are determinations for a 5.0 volt and 7.5 voltbattery source accomplished typically using a voltage doubler andtripler respectively.

The next data point established in FIG. 3 is located in the next columnto the right (0.4 milliseconds) by locating the first data point whichhas a higher rheobase value b, than the previous value (2.5 volts, atwhich point the charge drained is 1.40 microCoulombs for a 0.88 Voltrheobase). The progression of data points then again goes down to thedata point at 2.5 volts. This process continues to the additionalcolumns to the right, each time going down to 2.5 volts.

When the data point at 1.6 milliseconds, 2.5 volts is reached (at whichthe charge drained is 8 microCoulombs for a 2.00 Volts rheobase), thenext data point will be the data point with the first rheobase valueabove 2.00 Volts. That point is 0.4 milliseconds, 4.25 Volts. Theprocess then continues to determine all of the data points which areshaded in FIGS. 3 and 4, the last of which is 1.6 milliseconds, 7.5Volts (at which the charge drained is the ventricular pacing lead 72microCoulombs for a 6.00 Volt rheobase). These shaded data points arethe only pairs of pulse stimulation amplitude and pulse width at whichthe device will be operated.

The optimal pulse amplitude/pulse width combinations illustrated inFIGS. 3 and 4 have been entered into a single table in FIG. 7. Since itmay be noted from the table of FIG. 7 that there are several occasionsin which the variation of the rheobase b from one pair of pulseamplitude/pulse width combinations to the next is relatively small (forexample, from 0.83 Volts to 0.88 Volts), it may be desirable toeliminate those data points which do not produce a minimal variation inthe rheobase. Those skilled in the art will readily appreciate how toaccomplish this expeditiously.

A first implementation of the present invention would be to have thepacemaker permanently programmed to pulse amplitude/pulse widthcombinations based upon a standard chronaxie c value. For example, thetypical value of the chronaxie c is 0.4 millisecond. By selecting thisvalue, the values of the pulse amplitude/pulse width combinations forthe pacemaker can be determined.

A second implementation allows the physician to program the value of thechronaxie c into the pacemaker system. This can be accomplished in twodifferent ways. The first of these two ways is illustrated in FIG. 8,that depicts a process in which, after a prescribed value of thechronaxie is programmed into the programmer 38, the programmer is usedto calculate and select the optimal pulse amplitude/pulse widthcombinations, which are then downloaded into the pacemaker. Thepacemaker 30, like virtually all such devices at present, is capable ofmeasuring the lead/tissue impedance R. This value enables the values ofcharge delivered (Q) to be calculated as shown, for example, in FIG. 3.Furthermore, the pacemaker 30 can also measure its own internal currentdrain from battery 66 as monitored by the current measurement circuit67.

In block 110, the physician enters a value for the chronaxie c into theprogrammer 38 and based upon this value the programmer 38 will calculateall of the values in the rheobase table (e.g., FIG. 4) in block 112.Also in block 112, the programmer 38 calculates the optimal values ofthe pulse amplitude/pulse width combinations. The optimal values of thepulse amplitude/pulse width combinations are then telemetered into thepacemaker 30 in block 114, where they are programmed into memory (suchas RAM 82).

The second way in which the physician can program the value of thechronaxie c into the pacemaker system is illustrated in FIG. 9. In block120, the physician selects a value for the chronaxie c from a pluralityof values which may be chosen using the programmer 38. The selectedvalue of the chronaxie c is then telemetered into the pacemaker 30 inblock 122. In block 124, the pacemaker 30 uses the selected value of thechronaxie c to select a corresponding one of a plurality of differentsets of optimal values of the pulse amplitude/pulse width pairs (whichare typically stored in the off-board ROM 84), with the selected set ofoptimal values being programmed into memory (RAM 82).

A third implementation uses the programmer 38 to determine a value forthe chronaxie c. This can be accomplished in two different ways. Thefirst is illustrated in FIG. 10, which depicts a process in which theprogrammer 38 calculates the value of the chronaxie c, then the tablevalues, and then selects the optimal pulse amplitude/pulse widthcombinations, which are then programmed into the pacemaker. In block130, the programmer 38 operates the pacemaker 30 to determine a firstpulse amplitude threshold V1 at a first pulse width d1. In block 132,the programmer 38 operates the pacemaker 30 to determine a second pulseamplitude V2 at a second pulse width d2.

These two data points are then used by the programmer 38 in block 134 tosimultaneously solve two instances the following formula for the actualvalue of chronaxie c:

c=(PA 1-PA 2)/(PA 2/PW 1-PA 1/PW 2) where  (6)

PA1=a first pulse amplitude at a first preselected pulse width (PW1)when capture is first obtained; and PA2=a second pulse amplitude at asecond preselected pulse width (PW2) when capture first occurs. Basedupon this value of the chronaxie c, the programmer 38 will calculate allof the values in the rheobase table (e.g., FIG. 4) in block 136. Also inblock 136, the programmer 38 calculates the optimal values of the pulseamplitude/pulse width combinations. The optimal values of the pulseamplitude/pulse width combinations are then telemetered into thepacemaker 30 in block 138, where they are programmed into memory (RAM82).

The second way in which the programmer 38 may be used to determine avalue for the chronaxie c is illustrated in FIG. 11. In block 140, theprogrammer 38 operates the pacemaker 30 to determine a first pulseamplitude threshold V1 at a first pulse width d1. In block 142, theprogrammer 38 operates the pacemaker 30 to determine a second pulseamplitude V2 at a second pulse width d2.

These two data points are then used by the programmer 38 in block 144 tosolve equation (6) above for the actual value of chronaxie c. Theprogrammer 38 then telemeters the calculated value of the chronaxie c tothe pacemaker 30 in block 146. In block 148, the pacemaker 30 uses theselected value of the chronaxie c to select a corresponding one of aplurality of different sets of optimal values of the pulseamplitude/pulse width combinations (again typically stored in theoff-board ROM or Ram 84), with the selected set of optimal values beingprogrammed into memory (RAM 82).

Referring finally to FIG. 12 and using c=0.4 milliseconds, an overviewof the method used by the improved pacing system of the presentinvention to automatically determine and maintain the optimal pacingenergy is illustrated. The method illustrated in FIG. 12 is thepreferred mode, and incorporates capture verification, issuance of asafety backup pulse, and automatic threshold determination. There arethree modules shown in FIG. 12: an initial threshold determinationmodule 150, a main pacing operation module 152, and a periodic thresholddetermination module 154. The initial threshold determination module 150operates whenever the pacemaker 30 is initially implanted in a patient,or when the pacing system has been reset and must reinitialize itsoperation.

The pacemaker 30 sets an initial energy level in block 160, and thenpaces the patient in block 162. Capture is monitored in block 164, andif capture is verified, the pacing energy level is decreased in block166 by selecting values for the pulse amplitude/pulse width pairs whichare associated with the next lowest rheobase value selected from FIG. 7.After decreasing the pacing energy level, the device returns to pacingin the block 162, after which it attempts to verify capture in block164. If capture is not verified in the block 164, the process isdiverted to block 168, where a safety backup pulse is delivered (safetybackup pulses are typically delivered at 4.5 Volts to ensure capture).

Following delivery of a safety backup pulse in block 168, the pacingenergy level is increased in block 170 by selecting values for the pulseamplitude/pulse width which are associated with the next highestrheobase value from FIG. 7. The pacemaker 30 then paces the patient inblock 172, and capture is monitored in block 174. If capture is notverified, the pacemaker 30 returns to the block 168. If capture isverified, the pacemaker 30 moves to block 176, where the patient ispaced, and capture is verified in block 178.

If capture is not verified in block the 178, the pacemaker 30 returns tothe block 168. If capture is verified (and thus has been verified fortwo consecutive beats at the selected values for the pulse amplitude andthe pulse width), the pacemaker 30 moves to block 180, where the pacingenergy level is increased by selecting values for the pulseamplitude/pulse width which are associated with the next highestrheobase value from FIG. 7 (for added safety margin).

It will be appreciated by those skilled in the art that in the initialthreshold determination module 150, the values for the pulseamplitude/pulse width which are associated with the lowest rheobasevalue which captures the patient's cardiac tissue are determined.Following this determination, a safety factor is added to the pacingoperation in the block 180 by setting the values for the pulseamplitude/pulse width to those values which are associated with the nexthighest rheobase value.

Optionally, the values associated with the second or even third higherrheobase value could also be used to increase the safety factor. Such anadded increase (beyond increasing one rheobase value) is believed to beunnecessary in the preferred embodiment since the pacemaker 30 has bothcapture verification and safety backup pulse capability. The operationof the initial threshold determination module 150 is used when thepacemaker 30 is first implanted, or when a system reset occurs.

The normal pacing operation of the pacemaker 30 is contained in the mainpacing operation module 152, which may be entered from the block 180 inthe initial threshold determination module 150. The pacemaker 30 pacesthe patient in block 182, and then monitors capture in block 184. Ifcapture is verified in the block 184, the system moves to the periodicthreshold determination module 154, where in block 186 a determinationis made whether or not eight hours has elapsed since the last time thatthe patient's threshold was checked in the periodic thresholddetermination module 154.

If it has been less than eight hours since the patient's threshold waslast checked, the system returns to the block 182 in the main pacingoperation module 152. The three blocks of pacing in the block 182,verifying capture in the block 184, and checking to determine whethersufficient time has elapsed to check threshold in the block 186 comprisethe normal operating routing of the pacemaker 30. The procedure to befollowed if eight hours have elapsed since the patient's threshold waslast checked will be discussed below in conjunction with the discussionof the periodic threshold determination module 154.

If capture is not verified in the block 184, the process is diverted toblock 188, where a safety backup pulse is delivered. Following deliveryof a safety backup pulse in block 188, the pacing energy level isincreased in block 190 by selecting values for the pulse amplitude/pulsewidth which are associated with the next highest rheobase value FIG. 7.The pacemaker 30 then paces the patient in block 192, and capture ismonitored in block 194. If capture is not verified, the pacemaker 30returns to the block 188. If capture is verified, the pacemaker 30 movesto block 196, where the patient is paced, and capture is verified inblock 198.

If capture is not verified in the block 198, the pacemaker 30 returns tothe block 188. If capture is verified(and has again been verified fortwo consecutive beats at the selected values for the pulse amplitude andthe pulse width), the pacemaker 30 moves to block 200, where the pacingenergy level is increased by selecting values for the pulseamplitude/pulse width which are associated with the next highestrheobase value from FIG. 7. This adds a safety factor to the pacingoperation by setting the values for the pulse amplitude/pulse width tothose values which are associated with the next highest rheobase value.

As mentioned above, a threshold test is run periodically (every eighthours in the preferred embodiment) in order to determine whether thestimulation threshold has decreased in the periodic thresholddetermination module 154. If it is determined in the block 186 thateight hours have elapsed since the last time that the threshold waschecked, operation of the pacemaker 30 moves to block 202, where thepacing energy level is deceased by selecting values for the pulseamplitude/pulse width which are associated with the next lowest rheobasevalue from FIG. 7.

After decreasing the pacing energy level, the device returns to pacingin the block 204, after which it attempts to verify capture in block206. If capture is not verified in the block 206, the process isdiverted to block 208, where a safety backup pulse is delivered.Following delivery of a safety backup pulse in block 208, the pacingenergy level is increased in block 210 by selecting values for the pulseamplitude/pulse width which are associated with the next highestrheobase value from FIG. 7. The pacemaker 30 then paces the patient inblock 212, and capture is monitored in block 214. If capture is notverified in the block 214, the pacemaker 30 returns to the block 208. Ifcapture is verified, the pacemaker 30 moves to block 216, where thepatient is paced, after which capture is verified in the block 218.

If capture is not verified in the block 218, the pacemaker 30 returns tothe block 208. If capture is verified(and has again been verified fortwo consecutive beats at the selected values for the pulse amplitude andthe pulse width), the pacemaker 30 moves to block 220, where the pacingenergy level is increased by selecting values for the pulseamplitude/pulse width which are associated with the next highestrheobase value from FIG. 7. This adds a safety factor to the pacingoperation by setting the values for the pulse amplitude/pulse width tothose values which are associated with the next highest rheobase value.The operation then returns to the block 182 in the main pacing operationmodule 152.

Although an exemplary embodiment of the improved pacing system of thepresent invention has been shown and described with reference toparticular embodiments and applications thereof, it will be apparent tothose having ordinary skill in the art that a number of changes,modifications, or alterations to the invention as described herein maybe made, none of which depart from the spirit or scope of the presentinvention. All such changes, modifications, and alterations shouldtherefore be seen as being within the scope of the present invention.

What is claimed is:
 1. An implantable cardiac stimulation device capableof providing pacing therapy, the implantable cardiac stimulation devicecomprising: a first memory that is capable of storing a plurality ofpulse amplitude/pulse width combinations each of which is associatedwith a rheobase value and a stimulation pulse energy content, whereineach of the pulse amplitude/pulse width combinations has the lowestbattery charge drain of any possible pulse amplitude/pulse widthcombination having at least the rheobase value of that particular pulseamplitude/pulse width combination; control circuitry, operativelyconnected to the first memory, that is operative to select a pulseamplitude/pulse width combination; a stimulation pulse generator that isoperative to generate stimulation pulses at the selected pulseamplitude/pulse width combination, at prescribed times upon demand, tostimulate cardiac tissue; and an automatic capture detector whichdetects and provides an output indicative of whether the stimulationpulses obtain capture of cardiac tissue, the output of the automaticcapture detector being supplied to the control circuitry, the controlcircuitry causing the stimulation pulse generator to generatestimulation pulses at the selected pulse amplitude/pulse widthcombination if the stimulation pulses obtain capture of cardiac tissue,the output of the automatic capture detector causing the stimulationpulse generator to generate stimulation pulses at another pulseamplitude/pulse width combination from the first memory which isassociated with an incrementally higher rheobase value and stimulationpulse energy content to obtain capture with a safety margin.
 2. Animplantable cardiac stimulation device as defined in claim 1, whereinthe plurality of pulse amplitude/pulse width combinations arepermanently programmed into the first memory.
 3. An implantable cardiacstimulation device as defined in claim 1, additionally comprising:telemetry circuitry, coupled to the control circuitry, for establishinga telemetry link with an external programming unit, and for facilitatingthe transfer of the plurality of pulse amplitude/pulse widthcombinations into the first memory.
 4. An implantable cardiacstimulation device as defined in claim 3, additionally comprising: asecond memory, the second memory containing at least two differentpluralities of pulse amplitude/pulse width combinations, the externalprogrammer being operable to cause one of the two different pluralitiesof pulse amplitude/pulse width combinations to be supplied from thesecond memory to the first memory for storage therein.
 5. An implantablecardiac stimulation device as defined in claim 4, wherein the secondmemory is located in the implantable cardiac stimulation device.
 6. Animplantable cardiac stimulation device as defined in claim 4, whereinthe second memory is located in the external programming unit.
 7. Animplantable cardiac stimulation device as defined in claim 4, whereineach of the different pluralities of pulse amplitude/pulse widthcombinations is associated with one of a corresponding differentchronaxie values, a specific one of which may be selected by an operatorof the external programming unit.
 8. An implantable cardiac stimulationdevice as defined in claim 4, wherein each of the different pluralitiesof pulse amplitude/pulse width combinations is associated with one of acorresponding different chronaxie values, wherein the particular one ofthe plurality of sequences to be used is determined by threshold testingof the patient to determine pulse amplitude threshold at at least twodifferent pulse widths.
 9. An implantable cardiac stimulation device asdefined in claim 3, additionally comprising: means for calculating theplurality of pulse amplitude/pulse width combinations based upon a valueof chronaxie.
 10. An implantable cardiac stimulation device as definedin claim 9, wherein the value of chronaxie is entered by an operator ofthe external programming unit.
 11. An implantable cardiac stimulationdevice as defined in claim 9, wherein the value of chronaxie isdetermined by threshold testing of the patient to determine pulseamplitude threshold at at least two different pulse widths.
 12. Animplantable cardiac stimulation device as defined in claim 9, whereinthe calculation of the plurality of pulse amplitude/pulse widthcombinations based upon a value of chronaxie occurs in the externalprogramming unit, following which the plurality of pulse amplitude/pulsewidth combinations based upon the value of chronaxie entered by anoperator of the external programming unit are transferred into theimplantable cardiac stimulation device and stored in the first memory.13. An implantable cardiac stimulation device as defined in claim 9,wherein the value of chronaxie entered by an operator of the externalprogramming unit is transferred into the implantable cardiac stimulationdevice, where the control circuitry performs pulse amplitude/pulse widthcombination the calculation of the plurality of pulse amplitude/pulsewidth combinations and stores them in the first memory.
 14. Animplantable cardiac stimulation device as defined in claim 1, whereinrheobase value is calculated according to the formula b=V/(1+c/d) whereV is the pulse amplitude, d is the pulse width, and c is the tissuechronaxie.
 15. An implantable cardiac stimulation device as defined inclaim 1, wherein each pulse amplitude/pulse width combination isassociated with a particular rheobase value which differs from the nextlower rheobase value associated with a pulse amplitude/pulse widthcombination by at least a predetermined amount.
 16. An implantablecardiac stimulation device as defined in claim 1, additionallycomprising: a threshold detector which detects and provides an outputindicative of which of the plurality of pulse amplitude/pulse widthcombinations has the lowest battery charge drain while still obtainingcapture of cardiac tissue.
 17. An implantable cardiac stimulation deviceas defined in claim 16, wherein the output of the threshold detector issupplied to the control circuitry, the control circuitry causing thestimulation pulse generator to generate stimulation pulses at one of theplurality of pulse amplitude/pulse width combinations which has the sameor an incrementally higher rheobase value and stimulation pulse energycontent as the pulse amplitude/pulse width combination determined by thethreshold detector to have the lowest battery charge drain while stillobtaining capture of cardiac tissue.
 18. An implantable cardiacstimulation device as defined in claim 1, additionally comprising: asafety backup pulse generator which automatically causes the stimulationpulse generator to issue a high voltage safety backup stimulation pulseimmediately following a detection by the capture detector of a failureof any stimulation pulse to obtain capture.
 19. An implantable cardiacstimulation device as defined in claim 1, wherein the incrementallyhigher rheobase value and stimulation pulse energy content is the nexthighest rheobase value and stimulation pulse energy content from thefirst memory.
 20. An implantable cardiac stimulation device capable ofproviding pacing therapy, the implantable cardiac stimulation devicecomprising: a memory that stores a plurality of pulse amplitude/pulsewidth combinations each of which is associated with a rheobase value anda stimulation pulse energy content, each of which pulse amplitude/pulsewidth combinations has the lowest battery charge drain of any possiblepulse amplitude/pulse width combination having at least the rheobasevalue of that particular pulse amplitude/pulse width combination;control circuitry, operatively connected to the memory, that isoperative to select one pulse amplitude/pulse width combination from theplurality of pulse amplitude/pulse width combinations; a stimulationpulse generator that generates stimulation pulses at the selected pulseamplitude/pulse width combination, at prescribed times upon demand, tostimulate cardiac tissue; and a threshold detector which detects andprovides an output indicative of which of the plurality of pulseamplitude/pulse width combinations has the lowest battery charge drainwhile still obtaining capture of cardiac tissue, the output of thethreshold detector being supplied to the control circuitry, the controlcircuitry causing the stimulation pulse generator to generatestimulation pulses at one of the plurality of pulse amplitude/pulsewidth combinations which has the same or higher rheobase value andstimulation/pulse energy content as the pulse amplitude/pulse widthcombination determined by the threshold detector to have the lowestbattery charge drain while still obtaining capture of cardiac tissue;wherein the stimulation pulse generator generates stimulation pulses atanother pulse amplitude/pulse width combination from the memory which isassociated with an incrementally higher rheobase value and stimulationpulse energy content to obtain capture with a safety margin.
 21. Animplantable cardiac stimulation device as defined in claim 20, whereinthe incrementally higher rheobase value and stimulation pulse energycontent is the next highest rheobase value and stimulation pulse energycontent from the memory.
 22. An implantable cardiac stimulation devicecomprising: a memory that stores a plurality of pulse amplitude/pulsewidth combinations which are associated with different rheobase values,each of the combinations having the lowest battery charge drain of anypulse amplitude/pulse width combination having at least the rheobasevalue of that particular combination; a stimulation pulse generator thatgenerates stimulation pulses at a selected pulse amplitude/pulse widthcombination to stimulate cardiac tissue; and a capture detector whichdetects whether the stimulation pulses obtain capture of cardiac tissue,the capture detector causing the stimulation pulse generator to generatestimulation pulses at the selected pulse amplitude/pulse widthcombination if the stimulation pulses capture cardiac tissue, the outputof the automatic capture detector causing the stimulation pulsegenerator to generate stimulation pulses at another pulseamplitude/pulse width combination which is associated with anincrementally higher rheobase value from the memory to obtain capturewith a safety margin.
 23. An implantable cardiac stimulation devicecomprising: means for providing power to the stimulation device; meansfor providing stimulation pulses to cardiac tissue, such stimulationpulses have selectable pulse shapes; means for selecting a pulse shapefrom a plurality of selectable pulse shapes, each pulse being defined bya pulse amplitude and pulse width, forming a unique pair thereby, eachpair having associated therewith a specific rheobase value, a pulseshape being selected to provide the lowest battery drain relative toother pairs having the same specific rheobase value; and means fordetecting whether capture of the cardiac tissue has occurred and fortriggering the selection means to select a pair with an incrementallyhigher rheobase value to obtain capture with a safety margin.
 24. Animplantable cardiac stimulation device as defined in claim 23, furthercomprising: means for storing the plurality of selectable pulse shapesand their associated specific rheobase value, such plurality includingonly those pairs that provide the lowest battery drain relative to theother pairs having the same specific rheobase value.
 25. An implantablecardiac stimulation device as defined in claim 24, further comprising:means for sequentially selecting during periods of sustained detectedcapture, a pair with an incrementally lower rheobase value until captureis lost and selecting immediately thereafter the last pair whereincapture was obtained.
 26. An implantable cardiac stimulation device asdefined in claim 25, wherein the selecting means maintains cardiacstimulation using the selected last pair only if capture is obtained forat least two consecutive cardiac timing cycles.
 27. An implantablecardiac stimulation device as defined in claim 24, wherein theincrementally higher rheobase value is the next highest rheobase valuefrom the means for storing the plurality of selectable pulse shapes. 28.In an implantable cardiac stimulation device, a method of selecting astimulation energy level for the device, the method comprising: a)selecting an initial rheobase value to use to stimulate the heart; b)determining a corresponding pulse amplitude/pulse width combination forthe selected rheobase value, where the combination provides relativelylow battery charge drain for the rheobase value; c) determining whetherapplied pulses at the combination level capture the heart; d) selectinganother rheobase value if the applied pulses fail to capture the heart;e) repeating actions b) through d) to determine at least one rheobasevalue that results in capture of the heart; and f) generatingstimulation pulses at a value that is incrementally higher than therheobase value that results in capture of the heart to obtain capturewith a safety margin.